Rowland,
I noticed the same thing a couple months ago installing Jeff Robert's
new engine. Rotax now uses the .35 mm and our FS02 is .025 inches or
.7mm.
My A&P researched the difference with Lockwood and I did it through
Rotech.
We asked if we needed to change. They didn't care. The fuel pump
(mechanical) has been changed slightly and as usual Rotax never bothered
to explain the difference in flow and performance. We were told to
expect slightly higher fuel pressures with the new restrictor mounted on
the fuel split fitting. Basically if the fuel pressure is between 2.2
min and 5.8 max fine. No optimum is set by Rotax at max power at sea
level that I have found.
As for flow you will see no difference since the carbs are using the
fuel based on the need of the engine, not the pressure in the fuel line.
The float needle cuts off the flow to the carb. The flow back through
the return line will be that volume of fuel which is unused. Regardless
of pressure (provided it is within limits.) Only if the float needle
sticks open will you see a significant difference in flow (and of course
engine performance).
Other detail:
We did a test using the supplied Rotax restrictor on the new 912S vs our
old FS02. Fuel pressure dropped a bit with the FS02 but did not get
anywhere near the 5.8 max of the fuel pressure with both pumps running
and was above the 2.2 min with just the engine pump and when pulling the
fuel line off the engine pump and putting only the electric pump into
action it was still above the min. We saw a bit of an increase with the
smaller fuel restrictor of .3 psi. on our old hand held pressure gauge.
We were still getting 3.2 psi with the FS02 and 3.5 with the Rotax (as
measured with my really old gauge which was calibrated three years ago
to 3 psi or 83 in. H2O) so not much difference (Jeff's electric gauge
was about 4 psi with the Rotax engine pump and a bit lower on the
electric pump.
The manufacturer of the aircraft is responsible for his setting up of
the fuel system. If the FS02 works within limits, why change it.
Kerry at Lockwood is ambivalent about the difference. He has found some
builders who weren't using restrictors at all and the 912S ran fine on
high wing aircraft (Rans S 6). No report on the low wing aircraft, they
all had restrictors supplied by the kit manufacturer or Rotax. If it is
within limits, at full power, it is within limits.
In the US we don't have to do fuel flow tests, but to check you just
need to put a clear tube inline on the return side and look at the flow
or collect it into a measuring cup. If the pressure in the line with the
FS02 is only .3 psi different, then the higher pressure of the Rotax
orifice will flow faster through the orifice but the flow will be the
same as with the FS02. Pump volume makes a huge difference in return
flow. Nice of Rotax to change the pump slightly without providing tests
and results. Note that the wrong auxiliary pump (such as the automotive
low pressure style, like the Facet or Purilator 4104/5) will make a
difference as they don't provide the volume. Years ago Europa did SB 04
specifying the Facet 40106. Only if one installs a restrictive fuel
supply system by using a small diameter fuel selector, fuel filter,
gascolator, etc. will there be a problem. Keep the fittings and fuel
selectors as specified and the Europa fuel system works well. The LAA
requirement for excess fuel flow check is a great idea on an untried
engine/fuel system, but on a tried and true design it is just scary as
hell and a bit noisy and tedious. We just empty the pax side of the
tank, and run the engine checks on the main and note that the fuel
returns to the tank and then fiddle with the fuel flow on the Dynon or
EIS until close to 7 GPH. After about 15 minutes of running, you will
see a small amount of fuel in the reserve or pax side and have a fuel
flow that is fairly close for initial cruise checks.
Pretty stupid and lazy and not very scientific of us, but that is all we
have ever needed.
At my shop we check that the fuel pressure is in the range with only one
pump and does not exceed the max with both on. We have never had a
problem with the min pressure. Our US DARs don't care as long as full
power runs have been made and the kit system is installed as per the
manufacturer of the kit. If the kit manufacturer does not specify a
fuel system, a good DAR will want evidence of a full power run only, but
no excess fuel flow requirement is set. We are at sea level so the fuel
flow should be at its max with the prop set to 5200-5600 RPM at full
throttle. My DAR and FSDO are happy with that.
Best Regards,
Bud Yerly
----- Original Message -----
From: Rowland Carson<mailto:rowlandcarson@gmail.com>
To: Europa e-mail list list<mailto:europa-list@matronics.com>
Sent: Friday, June 21, 2013 1:19 PM
Subject: Europa-List: fuel return line restrictor
<rowlandcarson@gmail.com<mailto:rowlandcarson@gmail.com>>
While delving into the various Rotax manuals in pursuit of information
about the fuel manifold, I noted the dimensions of the restrictor in the
fuel return line - 0.35mm ID.
I have been considering the most elegant way to incorporate the
restrictor supplied by Europa (FS02) into my home-brewed alternative to
the Rotax manifold and so checked its dimensions. I find that the
restrictor hole will easily pass a number 71 drill bit, but barely
accepts the shank of a number 70 drill bit. So I deduce that the orifice
is about 0.7mm ID, ie twice the diameter of the Rotax one.
Why should Europa have supplied a different size of restrictor from
that called out by Rotax?
Looks as though the Rotax orifice will (in any otherwise identical
circumstance) allow the fuel pressure to build up slightly higher, and
return less fuel to the tank, than the Europa one. It might just make
the difference between pass and fail in a fuel flow test.
Has anyone had any issues traceable to this difference between the
Europa-supplied restrictor and the Rotax one?
I might consider machining up my own manifold (obviously I'm heading
into deep water with LAA engineering here) and if so, which size of
orifice should I incorporate? Should it be removable, rather than
integral, to allow fine-tuning of fuel pressure?
in friendship
Rowland
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